A hydraulic excavator is known as one example of a crawler mounted vehicle in which a plurality of actuators including a pair of left and right travel motors are simultaneously driven to perform the combined operation of traveling and any other at least one kind of operation. Such a hydraulic excavator comprises a lower travel body including a pair of left and right crawler belts to move the hydraulic excavator, an upper swing which is swingably mounted on the lower travel body, and a front mechanism consisting of a boom, an arm and a bucket. Various equipment such as a cab, a prime mover and a hydraulic pump are mounted on the upper swing to which is also attached the front mechanism.
A hydraulic drive system for use in that type crawler mounted vehicle may be constituted as a load sensing system which includes a pump regulator for controlling the pump delivery rate such that a delivery pressure of the hydraulic pump is held higher a fixed value than a maximum load pressure among the plurality of actuators, causing the hydraulic pump to deliver a hydraulic fluid at a flow rate necessary for driving the actuators, as disclosed in JP, A, 60-11706 by way of example. In the load sensing system, a pressure compensating valve is generally disposed upstream of each flow control valve. This permits a differential pressure across the flow control valve to be held at a predetermined value decided by a spring of the pressure compensating valve. By thus arranging the pressure compensating valve to hold the differential pressure across the flow control valve at the predetermined value, when plural actuators are simultaneously driven, the differential pressures across the flow control valves associated with all the actuators can be held at the predetermined value. It is therefore possible to precisely perform flow rate control for all the flow control valves irrespective of fluctuations in load pressures, allowing the plural actuators to be simultaneously driven at desired drive speeds in a stable manner.
In the load sensing system disclosed in JP, A, 60-11706, means for applying the pump delivery pressure and the maximum load pressure in directions opposite to each other is provided in place of the spring of each pressure compensating valve, so as to set the above predetermined value in accordance with the differential pressure therebetween. As mentioned above, the differential pressure between the pump delivery pressure and the maximum load pressure is held at the predetermined value by the pump regulator. Accordingly, the differential pressure between the pump delivery pressure and the maximum load pressure can be used, in place of the spring, to set the predetermined value of the differential pressure across each flow control valve. This also permits the plural actuators to be simultaneously driven in a stable manner as with the above case.
In the case of using the differential pressure between the pump delivery pressure and the maximum load pressure in place of the spring, when the hydraulic pump is saturated and the delivery rate runs short to supply the demanded flow rate, that differential pressure is lowered and the resulting lowered differential pressure is applied to all the pressure compensating valves, whereby the differential pressures across the flow control valves are now all held at a value smaller than the predetermined value during a normal mode. As a result, under such shortage of the pump delivery rate, the hydraulic fluid is prevented from being preferentially supplied to the actuator on the lower load side at a higher flow rate, so that the pump delivery rate is distributed at a ratio corresponding to the ratio of the individual demanded flow rates. In other words, the pressure compensating valves can develop a distribution compensating function even in a saturated condition of the hydraulic pump. With this distribution compensating function, the drive speed ratio of the plural actuators can properly be controlled even in a saturated condition of the hydraulic pump to enable the stable combined operation of the actuators.
Note that the pressure compensating valve installed to develop the distribution compensating function even in a saturated condition of the hydraulic pump, as mentioned above, is called "a distribution compensating valve" in this description for convenience of explanation.
However, the foregoing conventional hydraulic drive system has a problem as follows.
In the conventional hydraulic system, traveling speeds are basically controlled by operating control levers of the left-hand and right-hand travel motors. When lowering the traveling speed at the same operation amounts of the control levers, the delivery rate of the hydraulic pump is decreased to reduce the flow rate of the hydraulic fluid supplied to the left-hand and right-hand travel motors. Such a decrease in the delivery rate of the hydraulic pump is effected by, for example, lowering the revolution speed of the prime mover which drives the hydraulic pump. In other words, lowering the revolution speed of the prime mover reduces the available maximum delivery rate of the hydraulic pump. If the available maximum delivery rate is smaller than the flow rate demanded by operating the control levers, the hydraulic pump gets into saturation and the flow rate of the hydraulic fluid supplied to the left-hand and right-hand travel motors is decreased to lower the traveling speed.
With the above prior art of decreasing the revolution speed of the prime mover to reduce the delivery rate of the hydraulic pump and hence lower the traveling speed, during the combined operation of traveling and boom-up/down, the reduced delivery rate of the hydraulic pump is distributed at a ratio corresponding to the ratio of the demanded flow rates under the aforesaid action of the distribution compensating valves. This decreases the flow rate of the hydraulic fluid supplied to a boom cylinder, lowering the operation speed of the boom cylinder and degrading working efficiency. On the contrary, in an attempt of increasing the traveling speed during the combined operation of traveling and boom-up/down, the boom cylinder is speeded up in its operation, resulting in a fear that safety may decline dependent on the type of work. Thus, the conventional hydraulic drive system has had a problem that working efficiency and safety are degraded due to changes in the traveling speed during the combined operation inclusive of traveling.
An object of the present invention is to provide a crawler mounted vehicle for a hydraulic drive system with which traveling speeds can be changed without depending on the delivery rate of a hydraulic pump.